This invention relates to electric machines with superconducting field windings in general and more particularly to an improved sealing arrangement in such machine. Electric machines with rotors rotatably mounted in a stationary machine housing and containing a superconducting field winding to be deep cooled, one side of the rotor body being designed as a cylindrical shaft end piece and having, arranged around the shaft end piece, a nonrotating part of a connector head, there being provided between the shaft end piece and the connector head part at least one bearing and at least one sealing device, in which a transfer of a coolant takes place at the connector head, are known.
To cool a superconducting winding to be deep cooled in the rotor of an electrical machine, e.g., a turbogenerator, provisions for the transfer of a coolant suited for the purpose, particularly liquid or gaseous helium, between rotating and non-rotating machine parts must be made. Such transfer devices, also called couplers, are generally located on a connector head on the rotor face opposite the machine's drive system. It is through the couplers that the coolant is supplied to the rotor and discharged from it again. Designing such a coupler is particularly difficult, primarily with respect to minimizing thermal losses of the coolant loop for the field winding and with respect to small leakage rates of the cold coolant. Therefore, the couplers must be equipped with rotating packings which seal the coolant, in particular the liquid helium, well against the outside, on the one hand, and separate the inlet from the outlet side within the rotating system, on the other hand.
A machine with such a connector head is known, for instance, from "Advances in Cryogenic Engineering" Vol. 18, 1973, pages 372 to 381. This machine contains a rotor whose rotor body, on the side opposite the drive system, changes into a hollow cylindrical shaft end piece which projects into a connector head. The shaft end piece contains a central helium supply tube which is concentrically surrounded by two other tubes, between which an annular canal for the return flow of the helium from the rotor is formed. At its face, the shaft end piece is connected via a bearing to the connector head housing. The connector head is rigidly fastened to the machine housing enclosing the rotor.
The rotor of this machine performs both axial and radial oscillations, the amplitudes of which may amount to several hundred .mu.m when going through critical speeds such as during the starting phase of the machine or in the event of trouble. Therefore, the motor bearings in which the rotor is mounted within the stationary machine housing and the shaft end piece in the connector head housing must have appropriate clearance. This clearance must also be taken into consideration in the design of the packings required between rotating and nonrotating parts in the connector head. Consequently, correspondingly large coolant leakage rates on these packings cannot be avoided without complicating the design.
Further sealing problems arise if, in addition, a pump for the evacuation of the rotor interior is to be connected to the connector head. Such evacuation is necessary in order to be able to maintain a high insulating vacuum between hot and cold rotor parts over long operating periods. Due to this requirement, at least one sealing device capable of sealing off a large pressure difference must be provided on the connector head between a fixed and a rotating part of the evacuation line. However, because of the oscillations of the rotor and, hence, of its shaft end piece at the connector head, such seals are difficult to manufacture.